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><A
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>40.1. Overview</A
></H1
><P
>    PL/Tcl offers most of the capabilities a function writer has in
    the C language, with a few restrictions, and with the addition of
    the powerful string processing libraries that are available for
    Tcl.
   </P
><P
>    One compelling <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>good</I
></SPAN
> restriction is that
    everything is executed from within the safety of the context of a
    Tcl interpreter.  In addition to the limited command set of safe
    Tcl, only a few commands are available to access the database via
    SPI and to raise messages via <CODE
CLASS="FUNCTION"
>elog()</CODE
>.  PL/Tcl
    provides no way to access internals of the database server or to
    gain OS-level access under the permissions of the
    <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> server process, as a C
    function can do.  Thus, unprivileged database users can be trusted
    to use this language; it does not give them unlimited authority.
   </P
><P
>    The other notable implementation restriction is that Tcl functions
    cannot be used to create input/output functions for new data
    types.
   </P
><P
>    Sometimes it is desirable to write Tcl functions that are not restricted
    to safe Tcl.  For example, one might want a Tcl function that sends
    email.  To handle these cases, there is a variant of <SPAN
CLASS="APPLICATION"
>PL/Tcl</SPAN
> called <TT
CLASS="LITERAL"
>PL/TclU</TT
>
    (for untrusted Tcl).  This is the exact same language except that a full
    Tcl interpreter is used.  <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>If <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
> is used, it must be
    installed as an untrusted procedural language</I
></SPAN
> so that only
    database superusers can create functions in it.  The writer of a <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
>
    function must take care that the function cannot be used to do anything
    unwanted, since it will be able to do anything that could be done by
    a user logged in as the database administrator.
   </P
><P
>    The shared object code for the <SPAN
CLASS="APPLICATION"
>PL/Tcl</SPAN
> and
    <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
> call handlers is automatically built and
    installed in the <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> library
    directory if Tcl support is specified in the configuration step of
    the installation procedure.  To install <SPAN
CLASS="APPLICATION"
>PL/Tcl</SPAN
>
    and/or <SPAN
CLASS="APPLICATION"
>PL/TclU</SPAN
> in a particular database, use the
    <TT
CLASS="COMMAND"
>CREATE EXTENSION</TT
> command or the
    <TT
CLASS="COMMAND"
>createlang</TT
> program, for example
    <TT
CLASS="LITERAL"
>createlang pltcl <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></TT
> or
    <TT
CLASS="LITERAL"
>createlang pltclu <TT
CLASS="REPLACEABLE"
><I
>dbname</I
></TT
></TT
>.
   </P
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